The present invention concerns a method of laying of conduit at sea using one or more horizontal reels, each reel thus having a substantially vertical axis, and a vessel for performing such a method. The present invention is particularly, but not exclusively, concerned with laying of pipeline with reel vessels.
A reel vessel is a ship or barge specially designed to handle a pipeline that is wound onto a large and massive reel. Reel vessels are therefore relative large and massive and are used predominantly for laying flexible pipeline, rigid pipeline, and risers, and sometimes umbilical. Pipe-laying from a reel vessel has the great benefit of allowing the bulk of the welding, coating and inspecting of the pipeline to be carried out in factory conditions ashore to form a continuous length of pipe, which is then spooled onto a storage reel on the vessel, prior to being unspooled during pipe-lay operations. Unspooling pipe from a reel at sea requires a complex, specialized lay vessel, particularly when laying relatively long or larger diameter pipelines. An alternative method of pipeline laying is referred to as stove-piping, in which pipeline is formed by means of welding separate sections of pipe together on the vessel and welding such sections to the end of a pipeline leading to the ocean floor. Reel pipe-laying using existing arrangements is generally preferable to stove-piping, on economic grounds, for rigid pipelines having an outer diameter of 16″ (about 400 mm) or less.
In the case of laying pipeline from a reel vessel with a permanently fitted reel, when the reel is empty, the reel vessel has to move to an onshore facility to receive a new length of pipeline onto the reel. The onshore facility typically comprises an assembly line with at least one working station to join pipe sections end to end. After assembly of the pipeline to be laid, said pipeline is spooled (wound), onto the permanent reel of the pipelaying vessel. After completely filling the reel, the pipelaying vessel can return to continue the pipelaying process.
A drawback of the current reel pipelaying systems is that spooling is a time-consuming and inefficient process. Solutions offered in the prior art include providing a vessel with a vertical reel that is removably mounted on the vessel (see WO 2011/105894—Itrec) and having a vessel with multiple vertical reels (WO 00/66922—Coflexip). Vertical reel vessels (i.e. vessels arranged to support a reel with the axis of the reel being general horizontal) have the advantage of the reel having a relatively small footprint on the vessel and therefore offering the possibility of making relatively efficient use of space on deck. However, mounting a reel with the axis of rotation vertical (i.e. a so-called horizontal reel) has the advantage of stability, lowering the centre of gravity and potentially allowing for a simpler support structure for the reel, in view of the loads that the vessel needs to withstand as compared between a vertical reel and a horizontal reel.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved reel vessel. Alternatively or additionally, the present invention seeks to provide a more efficient and/or less time consuming method of laying pipeline from a reel vessel.